Everything to know about the shooting at a Californian college party that left 12 dead.

The gunman who opened fire on a crowd of mostly college students at western bar in California, killing 12 people including a sheriff’s deputy, has been identified as a US Marine Corps veteran.

Police say the shooting began on Wednesday at 11.20pm local time (6.20pm AEST) at the Borderline Bar and Grill in Thousand Oaks, which is about 64km west of Los Angeles.

The gunman was found dead at the scene. Ventura County Sheriff Geoff Dean named the suspect as Ian Long, 28, who likely took his own life.

Long appeared to have shot at random, using only a Glock .45-calibre handgun he had purchased legally. This weapon is designed to hold 10 rounds, plus one in the chamber, but Long used an extended magazine in this shooting.

There was no known motive, but Long had previous run-ins with the law.

One of the victims was identified as Alaina Housley, the 18-year-old niece of actress Tamera Mowry-Housley and husband Adam Housley.

“Our hearts are broken. We just learned that our niece Alaina was one of the victims of last night’s shooting at Borderline bar in Thousand Oaks,” Adam Housley told PEOPLEin a statement. “Alaina was an incredible young woman with so much life ahead of her and we are devastated that her life was cut short in this manner. We thank everyone for your prayers and ask for privacy at this time.”

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He said there were no preliminary indications the attack was terrorism-related but the investigation was ongoing. The FBI is assisting.

“It’s a horrific incident,” he said. “It’s part of the horrors that are happening in our country and everywhere.”

Dean identified the dead officer as Sergeant Ron Helus, a 54-year veteran of the police force who had planned on retiring soon. He is survived by a wife and son.

Helus and a California Highway Patrol officer were the first to arrive at the bar and went inside just before 11.30pm local time on Wednesday.

Witnesses reported a black-clad shooter hurled smoke bombs into the restaurant before he let loose a fusillade of shots.

Terrified customers began shattering windows in an attempt to escape. It was college night at Borderline, and the restaurant was filled with young people, including some from nearby California Lutheran University and from Pepperdine University in Malibu.

“I just started hearing these big pops – pop, pop, pop. There was probably three or four,” one young man told local television station KABC.

“The security guard … was down. And the gunman was throwing smoke grenades all over the place. I saw him point to the back of the cash register, and he just kept firing. I ran out the front door. I hear chairs being thrown out the window. People were trying to get out the window … There were probably 12 shots before I got out the front door.”

Other customers scrambled to find safety in other parts of the bar.

“There were people hiding in rest rooms,” Dean said. “There were people hiding in attics….It’s a horrific scene in there. There’s blood everywhere.”

The witness who spoke to KABC described the attacker as a bearded man in a black jacket and hat. Another witness said the gunman wielded what looked like a sub-machine gun.

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Dean said shots were still being fired when sheriff’s deputies arrived on the scene.

Squad cars and helicopters quickly swarmed the area. Sheriff’s Captain Garo Kuredjian said a SWAT team and officers from the Ventura and Simi Valley police departments converged on the scene, combing the area around the Borderline in search of the gunman in case he had fled the restaurant.

Ventura County Sheriff Geoff Dean said officers responded to a disturbance at the suspect’s home in April and described him as “acting irate”. A mental health specialist met with Long but he was not detained.

Dean said the crisis team that visited him felt the veteran may have been suffering from post-traumatic stress disorder.

Thousand Oaks was named the third-safest city in the US for 2018 by the website Niche.

“I’ve learned it doesn’t matter what community you’re in,” Dean told reporters when asked if he was surprised this happened in Thousand Oaks. “It doesn’t matter how safe your community is. It can happen anywhere.”

California Lutheran University, a private liberal arts school, is less than 8km away from the bar where the shooting attack took place and it is a popular venue with many students attending the university.

School officials said in a statement on its website that classes on Thursday were cancelled and the university community was invited to gather at the chapel.

Multiple students from Pepperdine University, a private school in Malibu about 40km south from the bar, were also at the venue and the school said on Twitter that it was working to identify and provide support to those students.

It was the third mass shooting in the US in under two weeks, six days after the death of two women at a yoga class in Tallahassee, Florida, and 12 days after a gunman killed 11 worshippers at a synagogue in Pittsburgh.

Jerry Nadler, the top Democrat on the House Judiciary Committee, said lawmakers would get to work on legislation including universal background checks when the House of Representatives convenes in January with a Democratic majority.

“We must find a way to stop the senseless, and many times preventable killings that are robbing our country of innocent lives,” he tweeted.

President Donald Trump, who has resisted a surge in calls for tougher gun controls since 17 students were shot dead at Stoneman Douglas High School in Florida earlier this year, said on Twitter that he had been “fully briefed on the terrible shooting in California”.

“Great bravery shown by police,” Trump tweeted. “God bless all of the victims and families of the victims.”